Hydrogen fluoride is industrially produced by reacting fluorite (containing CaF2 as a main component) with sulfuric acid (H2SO4) under heating (CaF2+H2SO4→2HF↑+CaSO4), and collecting the reaction mixture, which contains thus generated hydrogen fluoride (HF), in the form of a gas, followed by distillation.
A rude ore of fluorite may contain, in addition to calcium fluoride (CaF2) as a main component, impurities such as silicon dioxide (SiO2), calcium carbonate (CaCO3) and arsenic (As). Usually, a commercially available fluorite has already been purified, and those which are now used in the industrial production of hydrogen fluoride contain usually about 1% by mass of impurities such as silicon dioxide (SiO2), calcium carbonate (CaCO3) and arsenic (As). When hydrogen fluoride is produced by using such fluorite, silicon dioxide reacts with hydrogen fluoride to form silicon fluoride (SiF4). Silicon fluoride can be removed comparatively easily by distillation (silicon fluoride and hydrogen fluoride can be separated from each other by distillation, the former as a gaseous substance and the latter as a condensate). Calcium carbonate reacts with sulfuric acid to form calcium sulfate (CaSO4), carbon dioxide (CO2) and water (H2O). Calcium sulfate remains in a reacting furnace and is not mixed in hydrogen fluoride collected in the form of a gaseous substance, and water can be easily removed by simple distillation and also carbon dioxide can be removed comparatively easily by distillation. Therefore, both of them are not mixed in hydrogen fluoride. In contrast, arsenic, what kind of form which takes in fluorite is not clearly understood, reacts with hydrogen fluoride to form arsenic trifluoride (AsF3). Since a difference in a boiling point between arsenic trifluoride and hydrogen fluoride is small (a boiling point of HF is 19.5° C., a boiling point of AsF3 is 63° C.), it is difficult to completely remove arsenic trifluoride by distillation, and therefore arsenic trifluoride is mixed in hydrogen fluoride as a product.
Hydrogen fluoride is utilized as a starting substance for chlorofluorocarbon gases and fluororesins, which are fluorine chemical products, and is also utilized as an etching agent in the production of a semiconductor. When arsenic is mixed in hydrogen fluoride, there are problems that in the production of a chlorofluorocarbon gas, a catalyst can be poisoned, and that in the production of a semiconductor, arsenic can diffuse into a semiconductor device thereby exerting an adverse influence on operation of the device. Therefore, there is a demand for a high-purity hydrogen fluoride which contains as little arsenic as possible. In the process of producing a high-purity hydrogen fluoride in an industrial scale, a high-quality fluorite having an originally low content of arsenic is used as a reaction raw material, while a low-quality fluorite having a high content of arsenic is not used at present.